The blow-molding of plastic containers is well known and practiced worldwide. Blow-molding offers many advantages over other forms of container manufacturing.
Among these advantages are: the ability to produce containers at minimal cost and with minimal waste; the low start-up costs and mold making lead times in comparison to other manufacturing methods such as injection molding; the ability to produce irregular shaped and hollow containers; the ability to produce containers quickly and automatically; the ability to produce containers from a variety of materials having qualities suited to the specific application.
No other type of plastic processing offers the versatility, economy, and speed of blow-molding for producing plastic containers.
Blow-molds for producing such containers are commonly made of aluminum. Aluminum offers several advantages. Among those are: ease and economy of mold manufacture; light weight; efficient heat transfer. Because the process pressures and clamping forces during blow-molding are relatively low and mold erosion from the flow of molten plastic is not a factor, aluminum is amply strong and wear resistant for blow-molding, where other processes, such as injection molding, require the use of hardened steel. The relative softness of aluminum does however subject the mold to damage, such as during maintenance, and wear, such as when the mold halves do not correctly mate and cooperate. These factors, combined with the high production rates common to blow-molding, lead to the need for regular maintenance on and restoration of the aluminum molds. For instance, the matching parting faces of the mold halves must often be repaired or refaced. This refacing usually results in a reduction of the overall depth of the mold, measured from the back side of one mold half to the back side of the other across the parting face. Although a standard mold half depth of four and three-quarters inches is normally provided on new molds, that dimension is reduced with each such refacing.
Blow-molded containers having specially formed neck finishes are commonly employed for use with container closures. Neck finishes may be threaded for use with threaded closures, adapted for mating with snap-on closures, etc. It is common within a container blow-mold system to employ a main mold to form the container reservoir, and a neck block, or top block, to form the neck finish. Top blocks are also made to standard dimensions so that the parting face of the top block and main mold properly match. The main mold is usually adapted to interchangeably accept any standard top block for a particular container size or style. Top blocks and main molds are generally aligned visually by being loosely engaged, tapped into alignment such as with a mallet, then firmly affixed together. The back side of each mold half, and the back side, or heel, of each top block, are affixed to a planar mounting or back plate. Such tapping and rigid engagement, usually by steel bolts driven into threaded holes in the main mold, may cause damage and wear to the mold system, particularly when performed repeatedly as is common. Provided that the depth of the main mold half, from its parting face to its back side, is exactly equal to the depth of the top block half, from its parting face to its heel, the mold system can be properly aligned and effective molding can be performed therein. However, even though the top blocks and made molds are originally made to standard dimensions, extremely tight dimensional tolerances must be expensively met to prevent mismatching of the parting faces. When making a new mold system, it is more often economically advantageous to kit or match machine the mold halves to the top block halves to increase the likelihood of an acceptable parting line match.
During blow-molding, a parison of molten plastic is extruded between the open mold system, then the mold system closes to entrap the parison within the mold cavity. A hollow blow pin is inserted through the neck opening and into the parison where it inflates the parison with pressurized air to cause the parison to form to the shape of the container and neck finish cavity. The blow pin includes a hardened steel bushing, or shear bushing, having an annular blade, and each top block half includes a semi-circular hardened steel blade, or shear steel. When the mold is closed, the semi-circular blades and seals form annular orifices around the shear bushing. During molding, portions of the parison extending beyond the mold cavity are trapped between the parting faces of the mold halves and become unwanted flash. After the container is formed within the cavity and before the mold system reopens to release the blow-molded container, the shear bushing is retracted through the shear steel orifice. The shear bushing and shear steels are sized and shaped so that the retraction causes a shearing of the container opening through the neck, whose diameter is that of the shear bushing blade and shear steel orifice. After the molded container is removed from the mold, flash is removed by trimming in an automated process.
Misalignment of the main mold and top block parting faces will result in many problems during attempted molding. Among those are: unusual and hastened wear of the parting faces; excessive and untrimmable molding flash, concentrated and extreme pressures on areas of the mold; and improper shearing of the container opening.
The neck finish features are often the most detailed components of the mold system, having many features and requiring the highest degree of accuracy in manufacture. For instance, the mating halves of a threaded neck finish must match precisely and prevent mold flash to ensure that the container closure will properly fit onto the neck and seal the container opening. Interchangeable mold inserts are commonly employed within the top blocks to simplify and reduce the cost and time of making changes to the neck finish type. As a result, it is not so common to recondition or repair the top block itself as it is to recondition or repair a main mold which generally includes an integral cavity. This fact creates a problem and burden when main mold reconditioning or repair is required. Because the main mold depth will be reduced by such maintenance, the top block must also be reworked only to maintain an equal depth as the main mold, for proper alignment thereafter, or else a relief pocket must be precisely cut into the back plate to accommodate the new position of the top block's tail end. This causes an additional and unnecessary burden to the reconditioning process.